CBSE Class 12 Biology Organisms and Populations Question Bank

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Biology: Chapter 13 – Organisms and Populations (Question Bank)

Predators are prudent. Why? (1 mark)

Answer: They keep the prey population in check without causing its extinction. If a predator overexploited its prey, the prey would become extinct, and the predator would also die out.

Will the distribution of species be affected by a gradual increase in temperature? Say briefly. (1 mark)

Answer: Yes. The pattern of distribution of species will shift towards the poles (and to higher altitudes) as the temperature increases.

How is the orchid Ophrys pollinated by sexual deceit? (1 mark)

Answer: One petal of the flower resembles the female bee in size, colour and markings. The male bee is attracted to it and pseudocopulates with it, and in this way pollinates the flower.

What is the relationship between the egret and grazing cattle? Why? (1 mark)

Answer: Commensalism. The egret benefits, as it feeds on the insects disturbed by the cattle, while the cattle are neither benefited nor harmed.

Why is a female mosquito not considered a parasite? (1 mark)

Answer: It does not live in or on the body of the host. It only visits the host to take a blood meal.

What is brood parasitism? (1 mark)

Answer: In brood parasitism, one species lays its eggs in the nest of another bird and lets the host incubate them. For example, the cuckoo lays its eggs in the nest of a crow.

How is the liver fluke adapted to parasitism? (1 mark)

Answer: It has a flat body and suckers (adhesive organs) for attaching to the host.

Gause's competitive exclusion principle: where does it apply? (1 mark)

Answer: It applies to closely related species that compete for the same resources, when the resources are limiting. One of them is eliminated (competitively excluded).

How is competition avoided in nature? Give an example. (1 mark)

Answer: By resource partitioning. For example, when two species of warblers (or two species of insectivorous birds) feed on the same tree, they feed at different heights or at different times, so competition is avoided.

How have prey reduced the impact of predation? (2 marks)

Answer: Prey species have evolved various defences. Some are cryptically coloured (camouflaged) to avoid detection. Some are distasteful or poisonous, for example the monarch butterfly, which becomes distasteful to its predator (a bird) because it feeds on a poisonous plant as a caterpillar.

How have plants developed defences against predators? (2 marks)

Answer: Plants have morphological defences, such as thorns (in Acacia and cactus), and chemical defences. They produce chemicals such as cardiac glycosides (in Calotropis), nicotine, caffeine, quinine, strychnine and opium, which make the herbivores avoid them.

How are the four levels of biological organisation in ecology interrelated? (2 marks)

Answer: Similar organisms make populations. Populations of different species living in an area make communities. Communities along with the abiotic environment make the ecosystem, and the major ecosystems make up the biomes. So each level is made up of the levels below it: organisms → populations → communities → biomes.

How does temperature affect the distribution of animals? Name the two types based on their temperature tolerance. (2 marks)

Answer: Temperature affects the activity of the enzymes, and therefore the metabolic and other physiological activities of the organisms. This limits the range in which they can live. Based on their tolerance, organisms are eurythermal (they tolerate a wide range of temperature) or stenothermal (they tolerate only a narrow range).

How does water affect aquatic animals? Explain. (2 marks)

Answer: The pH of the water and its chemical composition affect aquatic animals. The salt concentration (salinity) varies from less than 5 parts per thousand in inland waters to more than 100 parts per thousand in hypersaline lagoons. Organisms can tolerate only a certain range of salinity, so this limits their distribution (as in freshwater and marine animals).

What is the importance of light for organisms? How do deep sea organisms get their energy? (2 marks)

Answer: Light is needed by the plants for photosynthesis. The quality of light and the photoperiod affect photosynthesis, and also the flowering, the reproduction, the foraging and the migratory activities of animals. In the deep sea, where there is no light, the organisms get their energy by scavenging and by predation (the food that sinks from the upper layers).

Which is the deepest-dwelling alga: red, green or brown? Why? (2 marks)

Answer: The red alga. Its pigment (phycoerythrin) can absorb the blue-green light that penetrates to the greatest depth, whereas the red and other longer wavelengths are absorbed in the upper layers of the water. So red algae can carry out photosynthesis in deeper water than the green and brown algae can.

What characteristics of soil determine the vegetation? How are these different at different places? (2 marks)

Answer: The composition of the soil, the grain size, and the percolation and water-holding capacity of the soil determine the vegetation, as do the pH, the mineral composition and the topography. These are different in different places, depending on the climate and the weathering process (the way the soil is formed).

What adaptations have parasites evolved? (2 marks)

Answer: The loss of unnecessary sense organs, the presence of adhesive organs or suckers to cling to the host, the loss of the digestive system, and a high reproductive capacity.

What do you mean by competitive release? (2 marks)

Answer: A species whose distribution is restricted because of the presence of a competitively superior species expands its distribution when the superior competitor is removed. This is called competitive release.

What are the roles of predators? (3 marks)

Answer:

1. They act as conduits for the transfer of energy to the higher trophic levels.
2. They keep the prey populations under control.
3. They help in maintaining the species diversity in a community, by reducing the intensity of competition among the competing prey species.

Give three examples of mutualistic relationships. (3 marks)

Answer:

1. Lichens: a relationship between an alga (or a cyanobacterium) and a fungus.
2. Birds and cattle (the cattle egret with the grazing cattle), or the mycorrhiza between a fungus and the roots of a plant.
3. The wasp and the fig tree (each fig species is pollinated by its own species of wasp).

Name the major biomes and the factors by which they vary. (3 marks)

Answer: The major biomes are the desert, grassland, forest (tropical, temperate and coniferous) and tundra. They vary in their precipitation, temperature and soil (and in the vegetation and animals that result from these).

What are the factors that lead to variation in the conditions of the habitat? (3 marks)

Answer: The factors are the abiotic components: soil, temperature, water and light. They also include the biotic components, such as pathogens, parasites, predators and competitors.

How does constancy of the internal environment help an organism? How do organisms achieve it with regard to temperature? Give an example. (3 marks)

Answer: A constant internal environment (homeostasis) allows all the biochemical reactions and physiological functions to proceed with maximal efficiency. Organisms regulate it by physiological means. For example, in humans, sweating reduces the body temperature when the external temperature rises, and shivering produces heat when it is cold.

What alternatives do organisms take if the stressful external conditions are localised? Give examples. (3 marks)

Answer: They can move away (migrate) or suspend their activities for a time. Migration: they move away to a distant place and return when the stressful period is over, as many birds do. Suspension: they suspend their activities for a brief period until the stressful conditions pass, for example hibernation in bears (to avoid cold) and aestivation in snails and fish (to avoid heat and dryness).

What is altitude sickness? How does the body overcome it? (3 marks)

Answer: At high altitudes, the body shows symptoms such as nausea, fatigue and heart palpitations, because there is not enough oxygen in the air (the atmospheric pressure is low). The body overcomes this by increasing the production of red blood cells, decreasing the binding capacity of haemoglobin, and increasing the breathing rate.

Give examples of adaptations to desert conditions shown by organisms. (5 marks)

Answer:

1. Cactus (Opuntia) has spines in place of leaves, to reduce water loss.
2. The stomata are deeply set in pits.
3. The leaves have a thick cuticle.
4. The plants use the CAM photosynthetic pathway (stomata open at night).
5. The flattened, green stems carry out photosynthesis in place of leaves.
6. Animals such as the kangaroo rat lose minimum water in excretion: they concentrate the urine, and they can meet their water needs from the metabolic water.

Give an example of biochemical adaptation shown by organisms. What are the behavioural adaptations shown by organisms? (5 marks)

Answer: Biochemical adaptations: thermophiles (organisms of hot springs) have thermostable enzymes; cold-water fishes have antifreeze proteins; the organisms of the deep sea have molecules that can withstand high pressure.
Behavioural adaptations: desert lizards bask in the sun when their body temperature falls and move into the shade or burrows when the temperature around them rises. Animals also migrate or hibernate to avoid stress.

Define population and population density. How is it estimated? What are the attributes that determine the population characteristics? (5 marks)

Answer: A population is a group of organisms of the same species living in a specific area at a given time. Population density is the number of individuals per unit area (or the biomass or the percentage cover, when the numbers are not a suitable measure). It is estimated by counting the individuals, or by other measures such as biomass or cover.
The attributes of a population are natality (birth rate), mortality (death rate), immigration and emigration, the sex ratio and the age distribution.

What factors contribute to the growth of a population? In what way does logistic growth differ from exponential growth? (5 marks)

Answer: Natality and immigration increase the population, while mortality and emigration decrease it.
Exponential growth takes place when the resources are unlimited, so the reproductive potential is highest, and the curve is J-shaped. In logistic growth, the resources are limited, so the population cannot grow beyond the carrying capacity (K) of the environment, and the curve is S-shaped.

Define adaptation. What is the adaptation shown by (a) whales, (b) the kangaroo rat, (c) the polar bear? Why do cold regions not have small animals? (5 marks)

Answer: Adaptation is any attribute of an organism (morphological, physiological or behavioural) that enables the organism to survive and reproduce in its habitat.

1. Whales have a thick layer of fat (blubber) below the skin, which prevents the loss of heat from the body.
2. The kangaroo rat in the desert needs no water to drink. It meets its water needs from the metabolic water (the oxidation of the fat in its food) and concentrates its urine to lose very little water.
3. The polar bear has thick fur and a layer of fat to keep warm, and it passes the severe winter in a state of hibernation.

Small animals have a larger surface area compared to their volume, so they lose heat faster than they can produce it. So small animals are not found in very cold regions.

Lions are natural predators. Can you compare this act with the indiscriminate killing of animals by humans for their leather, horns and so on? Comment. What can be done to create awareness about this? (4 marks)

Answer: No. A predator like the lion kills only to meet its need for food, and when the prey population falls, the predator population also falls. This is a natural process that keeps a check on the size of the population. Humans, on the other hand, kill for greed and not for basic needs. This can lead to the extinction of species.
We should realise that every species has a significant role in nature. Awareness can be created through campaigns, street plays, posters and school programmes, by strictly enforcing wildlife protection laws, and by refusing to buy products made from animals (such as skins, horns and ivory).

27. Give example of adaptation to desert condition shown by organisms. 5